This invention relates to pneumatic tires and particularly to the processing of the tire to assure dynamic uniformity of the tire. More particularly, the invention relates to an apparatus for reducing radial force variations of the tire.
In pneumatic tires, components such as beads, inner liner, tread and sheets or plies of rubberized cords utilized in the carcass of the belts are segmentally assembled. Such assemblage can result in a structural nonuniformity in pneumatic tires which cause radial force variations when the tire is rotated along a surface, such as a roadway. Radial force variations are those defects which result from "hard" and/or "soft" spots in the tire due to structural nonuniformities such as inconsistent wall thickness, ply arrangement and other deviations. When the radial force variations exceed an acceptable level, the ride of the vehicle can be adversely affected.
The above-described radial force variations are generally corrected on a tire uniformity correction machine, described, for example, in U.S. Pat. Nos. 3,574,973; 3,725,163 and 4,458,451, wherein the radial force variations are detected, measured and selected portions of tread rubber are removed by a pair of grinders, one located at each shoulder of the tire which effects a reduction in radial force variations.
In existing tire uniformity correction machines, a tire is mounted on a rotatable axle wherein the tire is inflated and rotated against a loadwheel. Radial force variations transmitted to the loadwheel are sensed by transducers, such as load cells wherefrom electrical signals representing the actual measured radial force variations are generated and sent to a computer. The signals are sorted and compared to predetermined lower and upper limits of correctable radial force variations wherein the computer makes a grind or no-grind decision by comparing the actual measured radial force variations to the lower and upper limits. If the measured radial force variations do not exceed the lower limit, no grinding is performed. If the measured radial force variations exceed the upper limit of radial force variations wherein the radial force variations cannot be corrected without excess removal of rubber, no grinding is done and the tire is also removed from the machine.
Grind instructions typically are generated when the measured radial force variations exceed the lower limit and are less than the upper limit. A pair of hydraulic cylinders are actuated wherein each hydraulic cylinder moves a corresponding rotary grinder to the shoulder of the tire. The rotary grinders remove material from selected areas of the shoulders to reduce the radial force variations to the acceptable limit.
The depth of grinding performed on the selected areas of the shoulders is dependent on the grind instruction generated by the computer and the preset position of the grinders relative to the shoulders of the tire. The preset position is generally referred to as the skim position wherein each grinding wheel is set to a reference location at a predetermined distance from the tire surface. The skim position of each grinding wheel can be altered by a mechanical adjustment of a shroud which supports the grinding wheel. However, the skim position of each grinding wheel is dependent on operator judgment as to the adjustment and the amount of contact between the grinding wheel and the shoulder of the tire.
It has been found the skim position of the pair of shoulder grinders can differ due to operator judgment error which can result in one grinder removing more rubber from one shoulder than is removed from the other shoulder. When more rubber is removed by one grinder, excessive force may be exerted on the tire resulting in an appearance blemish due to the tearing and ripping of the rubber across the selected area of grinding. When removal of large amounts of rubber is required to remove radial force variations, mispositioning of the grinders can result in one grinder grinding too far into the tire resulting in a blemished tire. In either case, the positioning of the grinders is generally not detected until a tire is blemished. A device and method is needed to detect, measure and compare the amount of grinding done by the pair of grinders in order to achieve an improved grinding of the tire and reduce the possibility of blemishing a tire.
The difference in the skim position settings of a pair of grinders can result in lower grind margins for a given tire. For example, if a tire is ground from 50 pounds peak-to-peak to 30 pounds peak-to-peak without blemishing the tire, the grind margin is 20 pounds. An increase in the grind margin would allow tires with higher radial force variations to be ground out without adversely affecting the tire. If the grinders are set at different skim positions, the grind margin of the "heavier" ground shoulder of the tire fixes the maximum force removable without blemishing the tire.
One way to measure the amount of grinding done by each grinder is to measure the amount of current used by the electric motor which operates the grinder. The current used by the motor can be monitored by an ammeter. However, the current utilized by each grinder motor is not only dependent on the grinding of the tire but also is dependent on the condition of the motor and other variable effects.
It is desirable to reduce the difference in the skim position settings of a pair of grinders in order to improve the uniformity of grinding of tires and to reduce blemishing of the tires.